Everyone is replaceable. Feeling valued is a separate cultural issue, but no company should have a major dependence on a single individual for survival.
Mitigating risk and ensuring continuity is a core tenant of good business.
it's true that very few people are truly irreplaceable and it's a good hedge for knowledge to be well distributed. at the same time, small companies rarely have the luxury of employing (and keeping busy) multiple senior architects. unless you're at a very large company, there's always going to be a bus factor somewhere - it's just the reality for 90% of small businesses. either that or you have lock-in to outside consulting companies and services and no one with deep technical insight - just someone who's a consultant coordinator.
i'm not saying it's good to be that bus factor (you're on-call basically 24/7), but it also has undeniable perks for the employee (not the employer).
in reality my bus factor is not that high. the nice thing about well-designed systems is that they seldomly need attention, and there's another dev here who's more than capable of bug fixing and maintaining what's already in place.
There are two types of bus factors though. It’s true that at a small company, you can’t share all your responsibilities with someone else, but you can make sure that if they need to you can be replaced by someone else (ie. if you get hit by a bus) by doing things like documenting your work. Alternatively, if you want to make sure your job is secure, don’t do that, but make sure you’re doing it knowingly.
Mitigating risk and ensuring continuity is a core tenant of good business.